1. Field of the Invention
The present invention relates to method and apparatus for forming an oxide thin film on a substrate by a metalorganic chemical vapor deposition (MOCVD) method.
2. Description of the Related Art
Recently, oxide superconductors have been discovered, such as YBa.sub.2 Cu.sub.3 O.sub.7 -.delta. and Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8, to which attention has been paid in this field of art.
The conventional superconductors are an alloy, or intermetallic compound, such as an Nb-Ti system and Nb.sub.3 Ge. The critical temperature (Tc) which is an indicator for the characteristic of the superconductors is 20 K. at least. For this reason, the superconductive characteristic is manifested only under a cooling condition using a very expensive liquid helium (4.2 K.) and the superconductors finds very limited applications.
On the other hand, some of the aforementioned oxide superconductors reveals the critical temperature of 100 K. and the characteristic of the superconductors is manifested under a cooling condition using liquid nitrogen (77 K.) which is industrially manufactured at low costs. It has been highly expected that the oxide superconductors find not only the conventional application but also new applications to, for example, electron devices such as ultra-high speed logic elements operating at 77 K.
In order for the oxide superconductors to be used for the industrial purposes, it is necessary that a defect-free oxide crystal can be reproducibly prepared whose composition is well-controlled. The oxide superconductor, if being applied particularly to the electronic devices such as the ultra-high speed logic elements, is required to provide a flat oxide thin film surface of a single crystal.
For the formation of a thin film of the oxide superconductor in the conventional technique, the physical vapor deposition (PVD) techniques have been used such as sputtering and electron beam evaporation. In accordance with these methods, the thin film can be formed on a substrate by an apparatus of a relatively simple design, but it is difficult to precisely control an amount of supply of every element independently, of which the oxide superconductor thin film is composed. Furthermore, since the amount of the aforementioned elements supplied is governed by the configuration of a sputtering target or an evaporation source, it is difficult to deposit an oxide superconductor of a desired composition, as a thin film, on the substrate reproducibly.
Appl. Phys. Lett. 52, 1743 (1988) and Japanese Patent Disclosure (KOKAI) 63-292524 disclose the technique for preparing an oxide superconductor by the MOCVD (metalorganic CVD), one of the CVD techniques, which utilizes the pyrolitic reaction of organic metal.
The MOCVD method is a chemical vapor deposition method as opposed to the aforementioned physical method and can precisely control a feed amount of every element independently of which an oxide superconductor is composed. It is, therefore, possible to provide enhanced control with which the oxide superconductor is formed as a thin film of a desired composition on a substrate.
However, this type of method has a problem as will be set forth below. That is, it has been reported that a thin film as deposited by the conventional MOCVD was composed of a polycrystalline or an amorphous phase, as opposed to the superconductive phase, for a growth temperature of 600.degree. C. or below to provide no superconductive characteristic and that the superconductive characteristic is revealed only after a deposited film was annealed at as high as about 800.degree. C. in air or in a pure oxygen atmosphere (Appl. Phys. Lett. 53, 1756 (1988).
Even if, however, such a thin film is obtained in the aforementioned steps, it is polycrystalline as deposited and the steps thus involved include a step for forming a superconductive phase of a different composition from that of the thin film deposited. In this case, the thin film obtained is very bad in surface flatness, failing to be used for the ultra-high speed electron devices as already set forth above.
If, in order to avoid such problem, the oxidation step and heat treatment step for recrystallization are omitted subsequent to forming a thin film, then no desired superconductive phase can be obtained unless the deposition temperature reaches at least 800.degree. C. or more. Even in this case, no better flat thin film surface is obtained.
The fact that the deposition temperature and the temperature of subsequent annealing are high will lead to a degraded film quality which results from the reaction of the thin film with the substrate.
That is, in accordance with the conventional MOCVD method, no desired superconductive phase can be obtained unless a high-temperature annealing is conducted in air, or in an O.sub.2 atmosphere, subsequent to forming the thin film. The omission of the high-temperature annealing requires a rise in the deposition temperature. These indicate that no adequate oxidation of the thin film progresses at lower temperature. It is, therefore, not possible to obtain the crystal phase of a desired superconductor at lower temperature unless the extent of oxidation is increased by some method or the other.
For the deposition of the oxide superconductor in the form of a thin film by the conventional MOCVD method, a mixed organic metal feed gas, together with an oxygen gas, is introduced into a reactor. In accordance with this method, if more oxygen is supplied into the reactor, some of it reacts with the organic metal vapor phase and is consumed in the formation of a compound other than a desired deposition film. For this reason, it becomes difficult to adequately supply oxygen on the surface of the thin film deposited. When such a deposition is carried out at lower temperature, the aforementioned tendency is liable to be more prominent.
In the case where the oxide superconductor is deposited in the form of a thin film with the use of the MOCVD method, it is necessary to heat a source container above room temperature because the vapor pressure of the organic metal as source is generally low. If the source container is heated as set forth above, the source is deposited on the inner wall of a piping extending from the source-container toward the reactor in the case where the piping is placed under room temperature condition. This causes that the source is supplied onto the substrate in the reactors, and that the passage of the piping is stopped up.
To solve these problem, a tape heater has primarily been wrapped around the feedstock containers and piping so that they are heated. It is, however, difficult to uniformly heat the whole piping system because valves, flow controllers, joints, etc. are intricately arranged in the course of the piping system. It is also difficult to heat a jointed portion of the pipe and a feed gas inlet of the reactor, by means of the tape heater, at the same temperature as that of the pipe. Some spot of the piping is not heated, causing the source to be deposited there. This problem remains to be solved.
For a plurality of source containers each containing a different organic metal, it is the usual practices to heat all these feedstock containers by the same heating source. Since, in this case, an optimum temperature for a requisite amount of respect source varies due to, for example, the difference of vapor pressure among the organic metal source, it has been difficult to control all the organic metal source at the optimum temperature. Furthermore, it has been impossible to control the supply of all the source gases at the same temperature as that of the source containers.